In recent years, as a demand for the Internet is explosively increased, an experiment to speed up communications has been made in an access network such as a “fiber to the home” (FTTH) or a backbone (or core) network). In one of next-generation networks, an optical transceiver used in an Ethernet such as a 40 G (gigabits)-Ethernet or a 100 G-Ethernet is actively developed. The “Ethernet” is a registered trademark.
In an optical receiver used in the optical transceiver, for example, a variable optical attenuator (VOA) may be used in order to prevent a light receiving element or an optical component such as a photodiode (PD) from being broken down due to an input optical power beyond a rated value. For example, the breakdown of the optical component such as the light receiving element can be prevented by feedback-controlling a loss amount (hereinafter, may be referred to as a “VOA loss”) of the VOA such that a current value according to the reception power of the PD does not exceed the rated value.
JP 4-207644 A and JP 2010-136195 A disclose a technology relating to an optical receiver.
JP 4-207644 A discloses a technology in which two VOAs having different VOA losses are provided at the front stage of the light receiving element and the optical power input to the light receiving element is controlled by switching these VOAs. In addition, JP 4-207644 A discloses a technology in which a comparator is provided to compare a current value according to the reception optical power of the light receiving element with a predetermined threshold and a hysteresis characteristic is set to the comparator to give the hysteresis characteristic to a switching of the VOA losses.
Meanwhile, JP 2010-136195 A discloses an optical receiver provided with a semiconductor optical amplifier (SOA). In the optical receiver disclosed in JP 2010-136195 A, a wavelength multiplex optical signal amplified by the SOA is separated (or demultiplexed) for each wavelength (or channel), an electrical signal (or strength) according to the optical power of each of the demultiplexed optical signals is detected, and a bias current of the SOA (or a gain of the SOA) is controlled according to the detection result.
The VOA used in the optical receiver may be controlled so that the VOA loss is minimized in a no-signal state (may be referred to as a light interruption state) that is a state where the optical signal is not input because the reception power of the optical receiver used for the VOA control is low. Upon increasing an input optical power to the optical receiver steeply due to a recover of the light interruption state, the VOA loss is controlled to be increased in order to prevent the breakdown of the light receiving element.
However, since a response of the control of the VOA is slow, optical overshoot, optical surges and the like would be occurred. They would cause the light receiving element to receive a signal having a power beyond the rated value. Thus a current beyond the rated value would flow in the light receiving element. Therefore, the light receiving element would be damaged. In the worst case, the light receiving element may be broken down.
In view of this, by preliminarily controlling the VOA loss to be, for example, a maximum value in the no-signal state or in a state determined substantially having no signal, it is possible to prevent a current beyond the rated value from flowing into the light receiving element even when the input optical power to the optical receiver is steeply increased. The no-signal state or the state determined substantially having no signal may be considered as, for example, a state where a current value according to the input optical power to the light receiving element is equal to or lower than a predetermined threshold. In this state, an alarm called a LOS (Loss Of Signal) alarm is asserted to indicate a loss of the optical signal. A current threshold to assert the LOS alarm may be called a LOS asserted threshold or a LOS asserted level.
However, when the VOA loss is preliminarily controlled to the maximum value as described above, it would occur a delay in time taken until the current value of the light receiving element exceeds the LOS asserted level and the input optical power reaches a level (hereinafter, referred to as a “LOS cancellation level”) at which the asserted LOS alarm is cancelled. For example, since a control response time of the VOA is taken in several microseconds (ms) order, there occurs a delay in time (hereinafter, referred to as a “LOS cancellation time”) taken until the LOS alarm is cancelled according to the order.
In this case, a LOS cancellation time less than 100 μs defined in a CFP (100 G Form-factor Pluggable) standard may be unsatisfied. In JP 4-207644 A and JP 2010-136195 A, there is no mention on a relation between breakdown prevention for the light receiving element and the LOS cancellation time.